Local probe microscopy, or scanning microscopy, is based on measuring a physical magnitude while scanning a tip at a very short distance from the surface of a sample. Local probe microscopy makes it possible to obtain an image of the surface topography of a sample with spatial resolution better than the resolution of an optical microscope.
There exist various techniques for local probe microscopy. Mention may be made in particular of the atomic force microscope (AFM), of the scanning tunnelling microscope (STM), and of the friction- or shear-force microscope (SFM).
The scanning tunnelling microscope has an electrically conductive tip that serves to collect electric current generated by the tunnel effect when the tip is brought to a very short distance (in the range 0 to 5 nanometers (nm)) from the surface of a conductive sample. An STM generally comprises tunnelling current regulator means based on imparting nano movements between the sample and the tip along the Z axis of the tip with the help of a piezoelectric ceramic, the Z axis generally extending transversely to the surface of the sample. The value of the position occupied along the Z axis for servo controlling the tunnelling current as a function of XY scanning of the tip is then representative of variation in the surface relief of the sample. The scanning tunnelling microscope makes it possible to provide an image of the surface topography with sub-nanometer resolution. An STM requires a conductive tip, generally a metallic tip, made of nickel or of tungsten. Nevertheless, an STM is limited to being applied to samples that are conductive.
An atomic force microscope (AFM) is based on using repulsion and attraction forces between atoms at the surface of the sample and atoms at the end of the tip, which is of nanometer size. An atomic force microscope generally comprises a lever having a tip made of silicon or of silicon nitride, possibly covered in a deposit of metal. In conventional manner, the movement of the tip is observed by measuring the deflection of a laser beam reflected on the lever of the tip. The distance between the end of the tip and the surface of the sample is controlled by very fine detection of attraction and repulsion conditions, so as to avoid any contact between the tip and the surface. An AFM can be used to observe a sample of any type.
A shear-force microscope (SFM) comprises a resonant or vibrating local probe, generally comprising a piezoelectric oscillator (or resonator, generally of tuning fork shape) made of quartz and having a fine tip fastened thereto. When excited at its resonant frequency f0 (in the range 15 kilohertz (kHz) to 30 kHz) by applying an electric signal to its terminals or by mechanical excitation, the resonator induces small-amplitude (≈1 nm) vibration of the tip transversely to the Z axis of the tip. When the vibrating point is brought up to the surface of a sample to within a distance of less than one hundredth of a nanometer, a modification is observed in the parameters of the resonance of the resonator under the action of friction forces and/or shear forces between the end of the vibrating tip and the surface of the sample. This modification of the resonator gives rise to damping of the amplitude of the resonator and to a shift in the resonant frequency or to a reduction in the quality factor (Q factor) of the resonator. The imaging mode consists in scanning the tip parallel to the surface of the sample and in measuring the amplitude of the current that results from the excitation, which is itself proportional to the amplitude of the mechanical oscillation of the branches of the resonator. Various types of tip are used for SFM: a tapering optical fiber or a metal tip. One of the limits of shear-force microscopy is that the distance between the tip and the surface of the sample is generally unknown, which distance usually lies in the range about 20 nm to about 100 nm. This distance is generally estimated by detecting the instant that contact is made between the tip and the sample, leading to destruction of the end of the tip, which spoils its spatial resolution. A shear-force microscope can be used to map the surface topography of any type of sample, but it provides a topographic image with spatial resolution in the XY plane that is relatively degraded because of the oscillations of the tip.
There also exist microscopes, known as multimode local probe microscopes, that combine various operating modes for local probe microscopy.
Thus, the publication by J-P. Ndobo-Epoy et al. “Shear-force microscopy with a nanoscale resolution”, Ultramicroscopy 103 (2005), pp. 229-236 describes a shear-force microscope having a resonant local probe comprising a tuning fork with a nickel tip adhesively bonded thereto. A first electronic circuit is connected to the two electrodes of the tuning fork in order to measure the amplitude of oscillation. A second electronic circuit is connected to the tip in order to measure the tunnelling current between the nickel tip and a gold sample. According to the authors of that publication, having one of the branches of the resonator loaded by the tip and by the adhesive gives rise to a considerable reduction in the quality factor of the resonator (Q≈100). Furthermore, that device does not make it possible to avoid contact between the tip and the surface of the sample, nor does it make it possible to calibrate accurately the distance between the end of the tip and the surface of the sample for distances of less than 20 nm, because of almost complete damping of the oscillation amplitude of the branches of the tuning fork.
Furthermore, the publication by Yeong Seo et al. “Electrostatic force microscopy using a quartz tuning fork”, Appl. Phys. Lett. 80, 4324 (2002) describes an electrostatic force microscope based on a resonant tuning fork and a nickel tip fastened to an electrode of the tuning fork. The tip is used either in contact mode with the surface of the sample in order to apply a constant electrostatic force locally, or alternatively at a constant distance of 50 nm in order to measure an electrostatic force between the end of the tip and the surface of the sample.
The document by M. Woszczyna et al., “Tunneling/shear force microscopy using piezoelectric tuning forks for characterization of topography and local electric surface properties”, Ultramicroscopy 110, 877 (2010), describes a microscope with a local resonant probe comprising a tungsten tip fastened on a quartz resonator in the form of a two-branch tuning fork. The tuning fork is excited at its resonant frequency by mechanical excitation. Two electrodes on the branches of the tuning fork provide an electrical measurement of the amplitude of oscillation of its branches by using the piezoelectric effect that is naturally present in the quartz crystal. Those two electrodes are connected to a preamplifier to make it possible subsequently to amplify the signal relating to the shear forces between the tip and the surface of the sample. A third electrode electrically connects the tip to a current-to-voltage converter for measuring the tunnelling current between the tip and the surface of the sample, which is covered in a thin layer of gold or of diamond. That microscope makes it possible to measure independently the current due to the tunnel effect and the lateral shear force at a single point of the surface of the sample. Nevertheless, that configuration has the effect of drastically degrading the quality factor of the tuning fork (by a factor of 10), thereby reducing the sensitivity of the microscope in friction force conditions. The system is thus observed to become more rigid, which is harmful for regulating the distance between the tip and the sample, in particular for distances of less than about one-twentieth of a nanometer. In addition, that device does not make it possible to calibrate accurately the distance between the tip and the sample, which distance is merely estimated. It is true, that Karrai et al. (Phys. Rev. B 62, 13174, 2000) disclose topographic measurement revealing atomic-scale roughness of a graphite sample under a vacuum in constant tunnelling current mode, with a tuning fork that is excited at its resonant frequency. Nevertheless, that prior art system requires an evacuated environment, with its operation under atmospheric pressure being greatly degraded.
Furthermore, near field microscopy can advantageously be coupled with various analysis techniques. In particular, the tip enhanced Raman spectroscopy (TERS) or nano Raman technique relates to coupling a Raman spectroscopy apparatus with a local probe microscope having a tip made of noble metal or covered in a noble metal. Enhancement is observed of the Raman signal emitted locally at a point of the surface of a sample when the excitation laser beam of the Raman spectrometer is focused on the end of the tip of the near field microscope that has been brought to within a few nanometers of the surface of the sample, with this being due to local amplification of the electromagnetic field. The tip-to-sample distance is generally regulated by an AFM, but topographical resolution is then degraded, given the layer of metal deposited on the tip, or else by means of an STM, however the need to perform regulation on the tunnelling current makes it possible to use TERS analysis on conductive samples only. This means that the TERS technique is very difficult to implement.
It is desirable to develop a local probe microscope, in particular for TERS applications, in which the distance between the end of the tip and the surface of the sample lies in the range 0 to about 20 nm, with this distance being controlled and calibrated. A first difficulty is bringing the tip up to the surface of the sample to within a very short distance of only a few nanometers. Another difficulty is controlling this very short distance during scanning by the tip. Yet another difficulty is avoiding contact between the end of the tip and the surface of the sample, since any contact is likely to damage the nanometer-size end of the tip. There does not exist a TERS Raman spectroscope that operates with a local probe of the friction-force type and that enables the distance between the end of the tip and the surface of a sample to be controlled and calibrated accurately within a range of distances extending from 0 to about 20 nm, and preferably less than 10 nm, without involving contact between the end of the tip and the surface.